1. Field of the Invention
The present invention generally relates to a method and apparatus for dispensing compressed gas from a storage facility to a vehicle. More particularly, the present invention relates to an improved method and apparatus for dispensing substantially greater amounts of compressed gas from a storage vessel. By dispensing greater amounts of stored gas, the improved method and apparatus reduces the need to repeatedly refill the storage vessel and thereby minimizes transportation costs associated with refilling.
2. Description of the Related Art
A variety of compressed natural gas (CNG) fueling stations have been proposed to dispense CNG into natural gas vehicles (NGVs). CNG fueling stations must provide cost efficient and convenient dispensing of CNG to overcome customer concerns that may hinder public acceptance of NGVs.
Several conventional fueling stations are described in a Gas Research Institute publication entitled "Technology Gap Analysis of CNG Refueling Systems, Final Report" (GRI-91/0371), September 1991.
A first conventional fueling station 9 is schematically illustrated in FIG. 1. Station 9 includes a dispenser 10, a priority panel 80, a compressor 70, a fueling nozzle 20, and three immobile CNG storage banks: low bank 30, medium bank 40, and high bank 60. Station 9 dispenses CNG to a NGV 50 in the following manner. First, CNG is transferred to NGV 50 from low bank 30. As the pressure of CNG of low bank 30 decreases and the pressure of CNG in NGV 50 increases, the flow rate decreases. At a predetermined minimum flow rate, the dispenser 10 switches to the medium bank 40 to utilize the CNG in medium bank 40. Similarly, the NGV 50 will be filled from the medium bank 40 until a predetermined flow rate is reached, at which point CNG is supplied from the high bank 60 to complete the fill. When the pressure of CNG in the low bank 30 or high bank 60 drops, a compressor 70 is used to refill storage banks 30, 40 and 60 to the desired pressures using the priority panel 80.
In conventional fueling station 9, storage banks 30, 40 and 60 typically require high capital costs associated with their installation and maintenance. Furthermore, dispensing limitations exist when the dispensing system switches between the different banks based on a minimum flow rate. If the minimum flow rate is set at a low value, there is greater extraction of stored CNG but slower fill rates. Conversely, if the minimum flow rate is set at a high value, there is less extraction of stored CNG and faster fill rates. Consequently, a compromise must be made between gas extraction and fill rates with the result that neither good extraction of stored CNG nor fast fill rates are achieved.
A second conventional fueling station 100 which includes a pressure booster 140 is illustrated in FIG. 2. NGV 50 is filled initially from a single storage bank 130 and the fill is completed by the pressure booster 140 to the required NGV pressure by drawing down CNG from storage bank 130 through the dispenser 160. A compressor 150 is used to refill the storage bank 130. Improved extraction of CNG and faster fill times are achieved through the use of a pressure booster. In addition, installation and maintenance costs may be lowered by dispensing the CNG from mobile tube trailer storage instead of immobile storage.
Although the pressure booster configuration improves extraction of stored CNG, the booster configuration is only able to extract approximately 58% of stored CNG at low pressure CNG storage. This is an economically inefficient use of stored CNG. In addition, there are increased transportation costs due to the low amount of gas extracted from storage since the trailer must be refueled frequently.